Field of the Invention
The present invention relates to a display device and a method of manufacturing the same, and more particularly, to a thin film transistor substrate of a display device with improved display quality.
Discussion of the Related Art
A Liquid Crystal Display (LCD) is a display apparatus that acquires a desired image signal by applying an electric field to a liquid crystal layer, which is introduced between a Thin Film Transistor (TFT) substrate and a color filter substrate and has anisotropic dielectric permittivity, and by controlling the quantity of light transmitted through the substrates by adjusting the strength of the electric field.
Examples of liquid crystal displays include an In-Plane Switching (IPS) liquid crystal display, which uses a horizontal electric field, and a Fringe Field Switching (FFS) liquid crystal display, which uses a fringe field.
Among these, the FFS liquid crystal display creates a fringe field by reducing the distance between a common electrode and a pixel electrode to be smaller than the distance between a thin film transistor substrate and a color filter substrate. The fringe field operates not only liquid crystal molecules between the common electrode and the pixel electrodes, but also liquid crystal molecules on both the pixel electrodes and the common electrode, which results in an improved aperture ratio and transmittance.
In the FFS liquid crystal display, as illustrated in FIG. 1, pixel electrodes 22 and a common electrode 24 are spaced apart from each other with a slit interposed therebetween. At this time, the distance between the pixel electrodes 22, which are located in each sub-pixel, differs from the distance between the pixel electrodes 22, which are located in respective sub-pixels on opposite sides of a data line 20 therebetween. As such, the length of an alignment layer 26, which is located in the inner area A1 between the pixel electrodes 22 located in each sub-pixel, differs from the length of the alignment layer 26, which is located in the edge area A2 between the pixel electrodes 22 located in the respective sub-pixels on opposite sides of the data line 20 therebetween. In this way, there occurs a difference in the resistance of the alignment layer 26 between the inner area A1 and the edge area A2, and in turn, the difference in the resistance of the alignment layer 26 causes different residual DC components between the inner area A1 and the edge area A2, resulting in the nonuniform electric fields of the respective areas.
Specifically, when the liquid crystal display is driven for a long time, or a unidirectional (positive or negative) electric field is applied to the liquid crystal layer for a long time, the electric field deviates upward or downward on the basis of a common voltage, and dopants in the liquid crystal layer are ionized to thereby become adsorbed on the alignment layer 26. That is, positive ions are absorbed on the alignment layer 26 that corresponds to a minus (−) electrode, and negative ions are adsorbed on the alignment layer 26 that corresponds to a plus (+) electrode. As the ions adsorbed on the alignment layer 26 are diffused to the liquid crystal layer, a residual DC voltage is generated. The residual DC voltage rearranges liquid crystal molecules of the liquid crystal layer even though no DC voltage is applied to the liquid crystal layer. Thereby, even when a new DC voltage is applied to the liquid crystal layer between the pixel electrodes and the common electrode in order to implement image transition, there occurs an afterimage defect whereby a previous image formed by the residual DC voltage remains. In particular, as illustrated in FIG. 2, because a higher residual DC voltage is generated in the edge area A2 in which the alignment layer 26 has a relatively long length than in the inner area A1, the electric fields of the inner area A1 and the edge area A2 become different from each other, and the afterimage defect is noticeable in the edge area A2.
In addition, a flicker defect, whereby momentary screen shaking is caused for approximately a few seconds, occurs until the residual DC voltage dissipates. At this time, as illustrated in FIG. 3, because the residual DC component in the inner area A1 almost completely dissipates after a time T1, the flicker defect occurs during the time T1. In addition, because the residual DC component in the edge area A2 in which the length of the alignment layer 26 is longer than in the inner area A1, dissipates after a time T2, which is later than the time T1 in the inner area A1, the flicker defect occurs during the time T2.